The airway epithelium is a critical site for binding and entry of the SARS and NL63 coronaviruses throughinteractions with their receptor angiotensin converting enzyme 2 (ACE2). ACE2, a terminal carboxypeptidase, is expressed on the apical surface of the epithelial cells that line conducting airways, as well as alveolar epithelial cells. ACE2 is released from the surface of epithelia into airway surface liquid (ASL) via cleavage by TACE (ADAM17) and other sheddases. This soluble ACE2 (sACE2) is catalytically active in ASL, although its native substrates and biologic funcfions are poorly understood. ACE2 appears to have other functions that include induction of epithelial cell signaling and defense function, and SARS-CoV S protein or SARS virus infection directly downregulate pulmonary ACE2 expression. Loss of pulmonary ACE2 function has been hypothesized to contribute to acute lung injury associated with SARS, sepsis, and acid aspiration. The overall goal of this project is to better understand how the expression and release of ACE2 is governed in ainway epithelia, using models of well-differentiated human airway epithelia and mouse models of SARS-CoV infection, and relate this to SARS-CoV pathogenesis. Our overall hypothesis is that changes in pulmonary ACE2 expression and activity contribute to SARS-CoV lung disease through reduced carboxypeptidase activity and changes in epithelial host defense signaling. There are 3 specific aims. Aim 1. Determine how airway epithelial cell ACE2 expression is affected by SARS-CoV infection. In this aim we will investigate the hypothesis that pulmonary ACE2 expression in the vascular and epithelial compartments is regulated by cellular differentiation state, age, and mediators of infiammation and infection. We will also investigate how S-protein engagement and TACE affect sACE2 release. Aim 2. Characterize the physiologic function of airway epithelial ACE2 during SARS-CoV infection. Here we emphasize detailed assessment of the enzymatic functions of ACE2 in the context of a local pulmonary RAS and kinin system and how they are perturbed during SARS-CoV infection and contribute to lung disease. Aim 3. Identify enzyme activity-independent ACE2 functions that modulate host defense responses. For this aim, we focus on ACE2 function in airway epithelia independent of its receptor properties and carboxypeptidase activity. ACE2 effects that directly modulate epithelial host defenses will be studied.